Fix non-escaped CStr

[rbran]

While running the code below, I noticed that binja would get possessed and output random strings:

use binaryninja::types::Type;
fn main() {
    binaryninja::headless::init();
    let my_type = Type::bool();
    let my_named_type = Type::named_type_from_type("MyName", &my_type);
    println!("my_named_type: |{my_named_type:?}|");
    binaryninja::headless::shutdown();
    println!("shutdown");
}

Output something like:

my_named_type: |typedef MyName
called `Result::unwrap()` on an `Err` valueBINJA_DIR/home/rbran/src/binaryninja-api/rust/src/headless.rsFailed to find libbinaryninjacore path!plugins;|
shutdown

On closer inspection, I noticed that was cause by this simple string being passed as Rust Str and not C Str, missing the null char.